Ffmpeg libx265 vs hevc_nvenc

I have an Nvidia graphics card, and am using the proprietary drivers. I looked into the ffmpeg H265 encoders available to me, and found hevc_nvenc. Using hevc_nvenc does in fact use the GPU to encode the video, which massively increases encode time, but the output filesize is considerably larger. For example: (input.mp4 is H264 and aac) ffmpeg -hwaccel cuda -i input.mp4 -c:v libx265 -c:a libopus -crf 26 libx265_output.mkv ffmpeg -hwaccel cuda -i input.mp4 -c:v hevc_nvenc -c:a libopus -crf 26 hevc_nvenc_output.mkv Filesizes are:

input.mp4             351M
libx265_output.mkv    134M
hevc_nvenc_output.mkv 360M

ffprobe shows both outputs as hevc encoded, and input as h264. So why does hevc_nvenc seem to perform so poorly? There must be something I'm missing.